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       clock_getres, clock_gettime, clock_settime - clock and time functions


       #include <time.h>

       int clock_getres(clockid_t clk_id, struct timespec *res);

       int clock_gettime(clockid_t clk_id, struct timespec *tp);

       int clock_settime(clockid_t clk_id, const struct timespec *tp);

       Link with -lrt.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       clock_getres(), clock_gettime(), clock_settime():
       _POSIX_C_SOURCE >= 199309L


       The function clock_getres() finds the  resolution  (precision)  of  the
       specified  clock  clk_id,  and,  if  res  is non-NULL, stores it in the
       struct timespec pointed to by res.  The resolution of clocks depends on
       the  implementation  and  cannot be configured by a particular process.
       If the time value pointed to by the argument tp of  clock_settime()  is
       not a multiple of res, then it is truncated to a multiple of res.

       The  functions clock_gettime() and clock_settime() retrieve and set the
       time of the specified clock clk_id.

       The res and tp arguments are  timespec  structures,  as  specified   in

           struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */

       The  clk_id argument is the identifier of the particular clock on which
       to act.   A  clock  may  be  system-wide  and  hence  visible  for  all
       processes,  or  per-process  if  it  measures time only within a single

       All implementations support the system-wide real-time clock,  which  is
       identified   by   CLOCK_REALTIME.   Its  time  represents  seconds  and
       nanoseconds since the Epoch.  When its time is changed,  timers  for  a
       relative  interval  are unaffected, but timers for an absolute point in
       time are affected.

       More  clocks  may  be   implemented.    The   interpretation   of   the
       corresponding time values and the effect on timers is unspecified.

       Sufficiently  recent versions of glibc and the Linux kernel support the
       following clocks:

              System-wide  real-time  clock.   Setting  this  clock   requires
              appropriate privileges.

              Clock  that  cannot  be  set and represents monotonic time since
              some unspecified starting point.

       CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
              Similar  to  CLOCK_MONOTONIC,  but  provides  access  to  a  raw
              hardware-based time that is not subject to NTP adjustments.

              High-resolution per-process timer from the CPU.

              Thread-specific CPU-time clock.


       clock_gettime(),   clock_settime()  and  clock_getres()  return  0  for
       success, or -1 for failure (in which case errno is set  appropriately).


       EFAULT tp points outside the accessible address space.

       EINVAL The clk_id specified is not supported on this system.

       EPERM  clock_settime()  does  not  have  permission  to  set  the clock


       SUSv2, POSIX.1-2001.


       On POSIX systems on which these functions  are  available,  the  symbol
       _POSIX_TIMERS  is defined in <unistd.h> to a value greater than 0.  The
       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)


   Note for SMP systems
       realized  on  many  platforms  using timers from the CPUs (TSC on i386,
       AR.ITC on Itanium).  These registers may differ between CPUs and  as  a
       consequence  these  clocks  may  return  bogus  results if a process is
       migrated to another CPU.

       If the CPUs in an SMP system have different clock sources then there is
       no way to maintain a correlation between the timer registers since each
       CPU will run at a slightly different frequency.  If that  is  the  case
       then   clock_getcpuclockid(0)   will  return  ENOENT  to  signify  this
       condition.  The two clocks will then  only  be  useful  if  it  can  be
       ensured that a process stays on a certain CPU.

       The  processors  in  an SMP system do not start all at exactly the same
       time and therefore the timer registers  are  typically  running  at  an
       offset.   Some  architectures include code that attempts to limit these
       offsets on bootup.  However, the code cannot  guarantee  to  accurately
       tune  the  offsets.   Glibc  contains  no provisions to deal with these
       offsets (unlike the Linux Kernel).  Typically these offsets  are  small
       and therefore the effects may be negligible in most cases.


       According  to POSIX.1-2001, a process with "appropriate privileges" may
       using  clock_settime().  On Linux, these clocks are not settable (i.e.,
       no process has "appropriate privileges").


       date(1),  adjtimex(2),   gettimeofday(2),   settimeofday(2),   time(2),
       clock_getcpuclockid(3),  ctime(3),  ftime(3), pthread_getcpuclockid(3),
       sysconf(3), time(7)


       This page is part of release 3.24 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

                                  2010-02-03                   CLOCK_GETRES(2)